Optical disk playback devices, for example, DVD (Digital Versatile Disk) players, have been spreading rapidly in recent years, and are being equipped with generalized functions that can play back other types of optical disks, specifically, functions that can play back CDs (Compact Disks), which have already become widespread. This kind of optical disk playback device is normally equipped with a function to determine the type of optical disk that is loaded in the equipment, and suitable playback processing or recording processing is conducted corresponding to the type determined.
In general optical disks, a long narrow convex part called a pit, which has a length corresponding to the recorded data, is arranged in a spiral by making a circumferential row from the center in the recording layer of a disk that is irradiated by laser light. When reading the recorded data from the optical disk, laser light is irradiated on this pit row.
A transparent substrate such as a resin is formed on the upper layer of this recording layer, the incident laser light is reflected by the previously described recording layer through this transparent substrate, and the recorded data is played back by using an optical detector to convert the reflected light to electrical signals.
The general method to determine whether an optical disk is a CD or DVD is to detect the thickness of this transparent substrate. Specifically, while the thickness of the transparent substrate in a CD is 1.2 mm, that in a DVD is half that thickness or 0.6 mm. Therefore, whether an optical disk is a CD or DVD can be determined by detecting the difference in this thickness.
FIG. 6 is a diagram to explain the general method to determine the type of optical disk. Indicated in the diagram are a disk motor 2 that clamps and rotates an optical disk 1, and an optical pickup 31 that focuses and irradiates laser light on a specific position on the optical disk 1, and that receives and converts that reflected light into electrical signals.
In the optical disk determination method indicated in FIG. 6, first, the optical pickup 31 is moved by an activator (not indicated in the diagram) at a constant velocity perpendicular to the surface of the disk while laser light is irradiated on the optical disk 1. By doing this, the focal position (focus) of the laser light is moved at a constant velocity perpendicular to the surface of the optical disk.
In conjunction with moving this focal position, several peaks are generated in the electrical signals (called the received light signals hereinafter) corresponding to the intensity of the reflected light converted by the optical pickup 31. For example, when the focal position is moved in the direction from the surface of the disk to the recording layer, the intensity of the reflected light first increases when the focal position reaches the disk surface and a first peak is generated in the received light signal based on the transparent substrate. Next, a second peak is generated in the received light signal when the focal position arrives at the recording layer. The distance between the disk surface and the recording layer is derived by using, for example, a time counter to measure the time interval generated between these first and second peaks. Whether an optical disk is a CD or DVD is determined by this measured time or distance.
In this regard, the peak value in the received light signal generated from reflection by the disk surface is extremely small compared to the peak value generated from reflection by the recording layer. Therefore, sometimes the effects of noise, etc. cause errors to be generated when detecting this extremely small peak value, and the disk determination operation becomes unstable.
FIG. 7 is a schematic block diagram indicating one example of a conventional received light signal detection circuit to detect light reflected from the surface of the optical disk and light reflected from the recording layer. In FIG. 7, the code 11 is a low-pass filter, the codes 12 and 13 are amplifiers, the codes 14 and 15 are peak retention circuits, the code 16 is a voltage division circuit, and the code 17 is a comparator.
The low-pass filter 11 is a filter to attenuate the high frequency noise component, which is outside a specified signal pass band, from the received light signal Srf.
The signals with the noise component attenuated by the low-pass filter 11 are input into the amplifier 12, and are amplified by a specified gain.
The amplifier 13 amplifies by a specified gain the signals from the amplifier 12 that are input via the capacitor C2.
The peak hold circuit 14 retains at a specified droop rate the maximum level peak of the amplified signal S13 that is input from the amplifier 13.
The peak hold circuit 15 retains at a specified droop rate the minimum level peak of the amplified signal S13 that is input from the amplifier 13.
The voltage division circuit 16 divides at a specified voltage division ratio the voltage of the maximum level of the amplified signal S13, which is retained by the peak holder circuit 14, and the voltage of the minimum level of the amplified signal S13, which is retained by the peak holder circuit 15. Normally, the voltage is divided to an intermediate level between the maximum and minimum levels.
The comparator 17 compares the level of the amplified signal S13 input from the amplifier 13 with that of the threshold signal S16 input from the voltage division circuit 16. If the level of the amplified signal S13 is greater than the level of the threshold signal S16, then the comparator 17 outputs a detection signal Sp of the logical value “1” that indicates detection of a peak.
The high frequency noise component of the received light signal Srf input into the low-pass filter is removed; and this signal is amplified by a specified gain by the amplifier 12, and is input into the capacitor C2. The amplified signal of the received light signal Srf, which has had the direct current component removed by this capacitor C2, is amplified by a specified gain by the amplifier 13, and is input respectively into the positive terminal of the comparator 17, the peak holder circuit 14, and the peak holder circuit 15.
The maximum level of the amplified signal S13 that is retained in the peak holder circuit 14 and the minimum level of the amplified signal S13 retained in the peak holder circuit 15 are divided by the voltage division circuit 16. The threshold signal S16, which has a level between these maximum and minimum levels, is input into the negative terminal of the comparator 17. The comparator 17 compares the levels of the amplified signal S13 with the threshold signal S16, and if the level of the amplified signal S13 exceeds that of the threshold S16, then a detection signal Sp of a logical value “1” is produced.
FIG. 8 is a diagram indicating examples of the waveforms of various parts in the optical disk determination circuit of FIG. 7.
In the received light signal Srf indicated in FIG. 8a, the peak A corresponds to the peak of the intensity of the light reflected by the disk surface, and the peak B corresponds to the peak of the intensity of the light reflected by the recording layer. As indicated in this diagram, the peak A caused by the surface of the disk is extremely small compared to the peak B caused by the recording surface.
Indicated in FIG. 8b is the waveform of the signal S13, wherein the peaks have been amplified by the amplifiers 12 and 13. The peaks are amplified by two amplifiers; the amplifiers output the saturated maximum level; and the maximum peak portions are flattened.
The maximum peak and minimum peak levels of the signal S13, obtained by amplifying the peaks of the received light signal Srf into a square wave shape, are retained respectively in the peak holder circuit 14 and the peak holder circuit 15. The voltage division circuit 16 generates the threshold signal S16, which has a nearly intermediate level between the two retained peaks. As indicated in FIG. 8c, the detection signal Sp is the high level when the level of this threshold signal S16 is less than the amplified signal S13, and is the low level when the level of this threshold signal S16 is greater than the amplified signal S13.
FIG. 9 is a diagram indicating the relationship between the signal waveforms input into the comparator of FIG. 7 and the logical threshold level of the comparator 17. In FIG. 9, the dotted line waveform of the threshold level indicates the logical threshold level that the input offset of the comparator 17 adds to the threshold signal S16. The logic of the detection signal Sp output from the comparator 17 is inverted if the amplified signal S13 exceeds this logical threshold level. Moreover, if no peak is generated in the amplified signal S13, the level of the threshold signal S16 is equivalent to that of the amplified signal S13 because the retention levels of the peak holder circuit 14 and the peak holder circuit 15 are equivalent to the level of the amplified signal S13. Consequently, in this case, the logical threshold level of the comparator 17 is only the input offset, and if a noise component that exceeds this input offset is added to the input of the comparator 17, there is the possibility that this will cause the detected signal Sp to be inverted to the logical value “1”. Specifically, in a system that sets up a threshold level for detecting peaks using a maximum peak level and minimum peak level retained by peak holder circuits, there is the problem that it is highly possible that an optical disk determination error will be made because noise is prone to cause operational errors during periods in which no peak is input.
In the optical disk determination circuit indicated in FIG. 7, differential action by the capacitor C2 causes the input signal level of the amplifier 13 to fluctuate after generating the peak. If a level fluctuation is generated during the non-peak period, the threshold signal S16 will also change because the maintenance levels of the peak holder circuit 14 and the peak holder circuit 15 will change. The timing whereby the detection signal Sp turns from the logical value “0” to the logical value “1”, as well as the period of the logical value “1” will change. Moreover, the timing whereby the detection signal Sp becomes the logical value “1” will become unstable, and the stability of the optical disk determination operation will be lost because the capacitance value of the capacitor C2 and the input impedance of the amplifier 13 will vary the size of this level fluctuation.
The present invention takes the related circumstances into consideration, and has the purpose of offering an optical disk determination circuit that, when determining the type of optical disk corresponding to the depth from the surface of the plane irradiated by the optical beam up to the data recording layer, can stably determine the type of optical disk by stably detecting the weak peak (pulse signal) of the received light signal corresponding to the intensity of the received light of the light reflected from the optical disk.